This invention relates to an image sensing apparatus for photographing a subject, by way of example, more particularly, to an image sensing apparatus having a tone control function preferrable for use in compensating for backlighting or the like.
When photographing a subject using a video camera or the like in a backlighted situation where illumination is intense, the tones contained in one picture (or frame) cover a wide range.
Since an image sensing apparatus using a solid-state image sensor as employed in a video camera or the like does not have enough dynamic range for converting all of these tones into video signals, clipping occurs above and below the brightness level. This is a phenomenon in which the subjects appears too bright and devoid of color or too dark, even black. Specifically, the tones of extremely bright portions and extremely dark portions cannot be expressed in one picture.
In the case of backlighted photography, the main subject becomes shadowed and appears very dark in comparison with the brightness of the background, and blackening of the subject occurs.
A conventional countermeasure to prevent the foregoing is to use a technique, referred to as backlighting control (BLC), through which a backlighting correction is performed by increasing the amount of exposure. A backlighting correction based upon this conventional technique will be described in detail with reference to the drawings.
First, the operation of BLC in an exposure adjusting mechanism using an iris will be described with reference to FIG. 1, which is a block diagram illustrating the construction of a backlighting correction circuit.
As shown in FIG. 1, light rays from a subject enter via an optical lens 10, the amount of light is limited by an iris 11, and an image is formed on an image sensor 12. A signal resulting from a photoelectric conversion and conforming to the state of the formed image is outputted by the image sensor 12 and converted into a video signal by a signal processing circuit 13, which delivers the video signal as an output.
The video signal is supplied also to an AE circuit 14, which generates an iris control signal conforming to the video signal. Upon receiving the iris control signal from the AE circuit 14 via an adder 15, a driver 16 generates a drive signal which drives an ig meter 17, whereby automatic adjustment of the amount of light by the iris is realized.
In the above-described approach for automatically adjusting the amount of light, the common practice is to make the adjustment in dependence upon the integrated value of the video-signal luminance level in one picture, and therefore emphasis is placed solely upon the average brightness of the entirety of one picture. As a consequence, in the case of backlighted photography, the adjustment of the amount of light incident upon the image sensor is carried out in dependence upon the background, which is extremely bright. The result is that the main subject becomes too dark, as a result of which the blackening phenomenon occurs.
In such a case, it is necessary for the photographer to issue a backlighting correction command to turn on the BLC function In the arrangement of FIG. 1, a BLC switch 19 is turned on (closed), whereby a correction signal generating circuit 18 supplies the driver 16 with a signal for widening the opening of the diaphragm. This signal arrives via the adder 15. As a result, the driver 16 drives the ig meter 17 in the opening direction to increase the amount of light incident upon the image sensor 12, whereby the backlighting correction is achieved.
By thus closing the BLC switch 19 to carry out the backlighting correction through this conventional technique, the image is changed from the BLC-OFF state (the state prior to the backlighting correction), shown in FIG. 2(a), to the BLC-ON state (the state after the backlighting correction), shown in FIG. 2(b). As will be appreciated from FIGS. 2(a) and (b), the background shows up well in the BLC-OFF state, but the subject is darkened and does not show up very well. By contrast, the tones of the subject are expressed and appear excellently in the BLC-ON state.
However, in the conventional image sensing apparatus described above, performing the backlighting correction increases the amount of light received by the image sensor, thereby causing the main subject to show up well, but the background appears washed out and cannot be made to appear. This may be understood from FIG. 2(b).
In other words, with the backlighting correction by the conventional technique, clipping occurs at a high or low level owing to the limitation upon the dynamic range of the image sensor, so that it is not possible to pick up the images of both the main subject and background well.